The CO (Carbon monoxide) potentiometer is an ECU adjustment to set the correct levels of CO at idle for those engines running open loop (i.e. no Oxygen / O2 / Lambda sensor). NOTE: The Turbo and NA CO pots are different!
Turbo CO pot (A100E6151F)
A 20 turn 10kohm potentiometer with a series 270R resistor connected between pins 1 and 3 and connected to pins A11 (ground) and B12 (ANASPRI) of the ECU. Turning the CO pot anti(counter) clockwise reduces the resistance and consequently the voltage read by the ECU but not apparently the voltage reported by ElanScan as this always seems to be 2V, however it may be different in service mode.
EDIT ADDED: Turn the screw clockwise to make it richer/raise the CO reading, anti(counter) clockwise to make it leaner/lower the CO reading. (Fredjohn Aug 2016)
NA CO pot (A100E6224F)
Unknown except all three pins are connected and used by the ECU via pins A11 (ground), B8 (ANASPRI) and C14 (5V) - if anyone has a spare, Geoff would be interested to see it!